A device for blocking a liquid flow through a pipe in one direction, said device being provided with a substantially spherical valve body, which can closingly cooperate with a substantially annular valve seat, which is positioned substantially transversely to the liquid flow. A guide element in provided for guiding said valve body in a path between a position in which said valve body is positioned near or against said valve seat (closed position) and a position in which the valve body is spaced from the valve seat by some distance (open position). In its open position said valve body can be positioned almost entirely outside the liquid flow. At least one outwardly extending projection is provided for effecting a reduction of the liquid pressure within the valve casing, at the side of the valve body remote from the valve seat.